Ideal power equation:

The ideal transformer as a circuit elementIf the secondary coil is attached to a load that allows current to flow, electrical power is transmitted from the primary circuit to the secondary circuit. Ideally, the transformer is perfectly efficient; all the incoming energy is transformed from the primary circuit to the magnetic field and into the secondary circuit. If this condition is met, the incoming electric power must equal the outgoing power.

Pincoming = IPVP = Poutgoing = ISVS giving the ideal transformer equation

Transformers are efficient so this formula is a reasonable approximation.

If the voltage is increased, then the current is decreased by the same factor.

The impedance in one circuit is transformed by the square of the turns ratio.[20] For example, if an impedance ZS is attached across the terminals of the secondary coil, it appears to the primary circuit to have an impedance of . This relationship is reciprocal, so that the impedance ZP of the primary circuit appears to the secondary to be